This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Background: Acute deviations in protein intake prior to quantifying protein kinetics in older humans may explain the controversy over the effects of older age on muscle protein synthesis and proteolysis rates. Objective: We hypothesized that an acute decrease in protein intake from habitual intake is associated with lower muscle protein synthesis and higher proteolysis rates, while an acute increase in protein intake from habitual is associated with higher muscle protein synthesis and lower proteolysis rates. Design: In 112 community-dwelling healthy 65-90 year old men, we quantified resting whole body 1,2-13C2-leucine kinetics, muscle mixed protein fractional synthesis rates (FSR), and muscle proteasome proteolytic enzyme activities after participants consumed 3 d controlled research meals (0.9-1.1 g protein/kg/d) that contained more or less protein than they habitually consumed.